A RF transmitting device, of which a digital cellular radiotelephone mobile subscriber unit is a convenient example, provides an appropriate setting for describing the need for the present invention. FIG. 1 is a diagram of cellular mobile subscriber units 110-112 operating in a cellular radiotelephone system 100. The cellular system 100 is designed to supply mobile telephone service using a "cellular" radio coverage plan. This plan divides a geographical area into a number of coverage areas or "cells" 103-109 with each cell being assigned a portion of the available radio channels. The channels used in one cell are again useable in spatially separated cells in the same coverage area 100.
Recommended minimum standards for the digital subscriber units 110-112 for use in the 800 MHz. Cellular operating band may be found in the Cellular System Dual-Mode Mobile Station-Base Station Compatibility Standard IS-54 (Revision A) published by the Electronic Industries Association in January 1991 which assures that the subscriber units 110-112 can obtain service in any cellular system 100 that meets the compatibility requirements of IS-54 (Revision A). For the sake of brevity, all subparagraphs (or sections) called out in this document refer to IS-54 (Revision A). Compatibility, is used in connection with IS-54 (Revision A), is understood to mean that any subscriber unit 110-112 is able to place and receive calls in any cellular system 100, and all cellular systems 100 are able to place and receive calls with any subscriber unit 110-112. To assure compatibility, it is essential that both radio system parameters and call-processing parameters be specified in the sequence of call-processing steps that the mobile and land stations execute to establish calls has been specified in IS-54 (Revision A) along with the messages and signals that are exchanged between the system provider and the subscriber units 110-112. The subscriber units 110-112 communicate with a distributed array of cell-site land stations, for example cell-site 113. The cell-site land station 113 is controlled by the cellular-system network control and switching equipment 115 that provides connection to the worldwide telecommunications network 117. A call between the land network 117 and the subscriber units 110-112 is routed via the cellular control system 115 to the cell site 113 that best serves the location of the subscriber units 110 through 112. A call in progress can proceed indefinitely as a subscriber unit 110, 111 or 112 moves from cell to cell throughout the coverage area 100 since it is automatically reassigned to an available channel with any new cell.
The transmitters in mobile subscriber units 110-112 must be capable of reducing or increasing output power on command from a cell-site land station 113 specifying eight output power levels per the aforementioned Cellular Standard IS-54 .sctn.2.1.2.2. The power level specified depends of the distance between the subscriber unit 110, 111 or 112 and the cell-site 113. For example, the output power level transmitted from subscriber units 110-112 increases as the distance between the subscriber units 110-112 and the cell-site 113 increases. However, the input power to the cell-site 113 from the subscriber units 110-112 is desired to be substantially equal regardless of the distance between the subscriber units 110-112 and the cell-site 113.
Cellular radiotelephones operating in a digital system, such as a time division multiple access (TDMA) signalling scheme described in IS-54 (Revision A), have a particular problem of generating a shaped transition between the minimum and maximum output power levels of the transmitter's power amplifier. In the TDMA system, data for one mobile subscriber unit is multiplexed in time with data for two other mobile subscriber units on the same channel. The format for a time frame of information sent by the mobile units 110-112 is described in .sctn.1.2.1. Each time frame includes two transmit, two receive and two idle time slots. Each subscriber unit 110-112 transmits a burst of information in a transmit time slot to the cell-site 113. Transmission of bursts of information by the mobile unit 110, for example, requires the mobile unit's transmitter to turn on and off at the beginning and end of each frame, respectively. In the TDMA system the maximum transition time between the on and off states of the mobile units transmitter is required to be three information symbol time periods per the aforementioned Cellular Standard IS-54 .sctn.2.1.2.1.2.
In general, reducing the transmission time between the on and off states of the transmitter increases the available time for transmission of information but has the adverse effect of increasing spectral splatter which interferes with adjacent and alternate channels assigned to other subscriber units. Adjacent and alternate channel interference specifications, -26 dbc and -45 dbc respectively, are described in the aforementioned Cellular Standard IS-54 .sctn.2.1.4.1.2. Increasing the transition time reduces the spectral splatter but shortens the available time for transmission of information. Therefore, a compromise is needed between acceptable spectral splatter and required time for data transmission. For a TDMA system, simply turning the transmitter on or off with a step function within the maximum transition time will not meet the spectral splatter requirement.
In the TDMA system, the transmitter is specified to operate over a wide dynamic range of 94.8 dB. Voltage controlled attenuators (VCA) typically used to control the magnitude of the power output signal have a dynamic range of 35 dB. Three cascaded VCAs may be used achieve a dynamic range of 105 dB thus meeting the dynamic range requirement of 94.8 dB. The problem with this approach is that additional signal processing and hardware is required to shape the transition between the on and off states using a digital signal source. Furthermore, it is advantageous to implement the VCAs on an integrated circuit (IC) but there are problems achieving the full dynamic range required due to the close proximity of cascaded VCAs on the IC.
Since the transmitter line up in a TDMA system is linear, the dynamic range of the transmitted signal may also be achieved by controlling various stages of the transmitter line up. The disadvantage of this approach is that additional control circuitry and signal processing would be required to control the shaped transition through the various stages thus increasing the cost and complexity of the transmitter.
For many situations, for which a digital cellular radiotelephone mobile subscriber unit operating in a TDMA system is merely an example, the prior art has not produced an apparatus or method of generating the transition region between the minimum and maximum output power levels of a RF power amplifier to meet the difficult requirement of minimizing spectral splatter while providing for the advantage of an economical transmitter design.
An apparatus and method of the present invention produces a shaped transition between an on and an off state of a radio frequency transmitter. The radio frequency transmitter is turned on. A data burst to be transmitted by the radio frequency transmitter is generated in response to the radio frequency transmitter being turned on. The data burst is filtered with a filter having a predetermined frequency response. The filtered data burst is amplified whereby the data burst is shaped according to the predetermined frequency response during the transition.